1. Field of the Invention
The present invention relates to a fuel-cell system and a method of estimating the nitrogen concentration on a fuel electrode of a fuel cell. More specifically, the present invention relates to the improvement of a technology for measuring or estimating the concentration of nitrogen present in a fuel electrode (anode) (also referred to as “anode nitrogen concentration” in the present specification) by transmitting nitrogen from an air electrode (cathode) to the fuel electrode through an electrolyte membrane.
2. Description of Related Art
One of the problems of a fuel-cell system is how to prevent reduction of the life of the fuel-cell system caused by an operation performed when the impurity concentration in the fuel electrode (anode) increases.
As a technology for solving such a problem, there has conventionally been a technology of obtaining the difference between a stack temperature and an ambient temperature when the operation of the fuel cell is stopped, and the difference between the stack temperature and the ambient temperature when the operation of the fuel cell is started, estimating the amount of time that has elapsed since the operation of the fuel cell has been stopped, as well as the concentration of nitrogen that has been transmitted from an air electrode (cathode) to anode via an electrolyte membrane, on the basis of temperature ratio (temperature difference when the operation of the fuel cell is started/temperature difference when the operation of the fuel cell is stopped), and limiting an output of a fuel-cell stack in accordance with the nitrogen concentration (see Japanese Patent Application Laid-Open No. 2004-172026, for example). This technology is capable of preventing the occurrence of excessive power generation in a state in which the impurity concentration in the anode is high.